This invention relates to a surface acoustic wave (SAW) filter and, more particularly, to a surface acoustic wave filter comprising a two-port surface acoustic wave resonator operable in three different longitudinal modes.
A surface acoustic wave filter of this type is disclosed in Japanese Patent Prepublication (A) No. 194,406 of 1988. This surface acoustic wave filter comprises as usual a pair of an input and an output interdigital transducer and a pair of grating reflectors on a piezoelectric substrate. A two-port surface acoustic wave resonator is thereby provided. In the manner which will later be described in greater detail, this surface acoustic wave resonator has a lower-frequency peak attenuation given by multiple reflection of a surface acoustic wave between the input and the output interdigital transducers and a higher-frequency peak attenuation resulting from internal reflection in the input and the output interdigital transducers. In effect, a frequency of the lower-frequency peak attenuation is brought into coincidence with another frequency of the higher-frequency peak attenuation in order to give a wide passband to the surface acoustic wave filter.
There are, however, several spurious resonance frequencies in and near the passband. These resonance frequencies give rise to adverse effects on widening the passband.
A little later than filing of a patent application for the above-referenced patent prepublication, a patent application was filed in Japan for a three-transducer surface acoustic wave filter. A counterpart United States patent application was issued to Takaya Watanabe and another as U.S. Pat. No. 4,910,426. According to this Watanabe et al United States patent, the surface acoustic wave filter comprises an apotized transducer. In a preferred embodiment, interdigital electrodes are used in an interdigital transducer in double electrode geometry with or without selective withdrawal or subsampling. Weighted finger electrodes are described. Moreover, details are described as regards how to make the invention. The specification of the Watanabe et al United States patent is therefore incorporated herein by reference.
An article is contributed by Yasushi Yamamoto, one of the present joint inventors, in the Japanese language to the Proceedings of the Institute of Electronics, Information, and Communication Engineers of Japan, Volume J76-A No. 2 (February 1993), pages 219 to 226, under the title in translation of "Composite Longitudinal Mode Resonator Type SAW Filters". In this article, Yamamoto refers to the surface acoustic wave resonator of the type described in the Japanese patent prepublication afresh as a composite longitudinal mode resonator (CLMR). Characteristics of the surface acoustic wave filters of such composite longitudinal mode resonator types are studied in detail.
In the meantime, Yasushi Yamamoto and Ryuuji Kajihara, the instant joint inventors, presented a paper in the Japanese language together with another to the 1993 Spring General Meeting of the Institute of Electronics, Information, and Communication Engineers of Japan, as paper No. A-340, under the title in translation of "SAW IF Filters for Digital Mobile Communication Systems (II)--Extension of Two-Dimensional Coupled Mode SAW Resonator Filters to Electrical Parallel Connection Type--". In this paper, the contributors have briefly revealed an outline of structure and results of experiments of a surface acoustic wave filter comprising a pair of two-port surface acoustic wave resonators connected electrically parallel on a piezoelectric substrate with double longitudinal modes used.